Does it have something to do with the US taking part in aiding reconstruction of the country after WWII? I've tried googling around for an answer, but every site I find simply states the outlet style, as well as voltage and frequency, as opposed to explaining why the outlet type is shared.

Just to be clear, I am asking what lead to the Japanese choosing to use Type A/B plugs & sockets. The spirit of the question is: What is the historical reason why if I buy a device in Japan it can clearly and easily plug into a US socket and vice versa.

I'm not asking about voltages. I'm not asking about AC/DC or current. I'm aware those things are different. I'm simply asking about the physical socket.

That's a common issue with search engines... when you're not looking for something that is expected but that your keywords are close to something else... I had the same issue when looking for the price of a bus (rather than the cost of riding a bus they expect us to search) - this was toatally impossible to find with any internet search engine.
– BregaladAug 14 '19 at 6:56

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Your question seems to assume that there was no electrical distribution and plugs in pre WWII Japan or that none survived the war and thus affords undue influence to post WWII US involvement in Japanese infrastructure. It is also important to point out that only some Japanese and US plugs are superficially similar.
– BOBAug 14 '19 at 14:14

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@BOB why do you think my question assumes that? Is it simply from my first sentence where I ask if it might have to do with WWII? It's not an unreasonable hypothesis, but I definitely have do not assume that Japan had no electricity before WWII. Also, can you point me to any Japanese plugs that aren't similar? I haven't seen any.
– spacetyperAug 14 '19 at 14:18

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@spacetyper I reread your question and I seem to have been reading more into it than is there concerning the US. As to the plugs themselves, they are not entirely identical from a design and manufacture perspective. I may appear to be splitting hairs here, but the standards that specify the Japanese plugs (JIS C 8303 Class I 3 Prong - 15 Amp and JIS C 8303 Class II 2 Prong - 15 Amp) differ from their US counter parts in the dimensional tolerances that are allowed. Additionally, Japanese plugs have no analogue for the US NEMA 1-15 polarised.
– BOBAug 14 '19 at 15:01

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@BOB That's when I like using search terms in quotes for "require this exact text" and search terms with a - before them for "skip anything with this text"
– Redwolf ProgramsAug 14 '19 at 17:37

2 Answers
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Japanese AC power outlets were first standardised in 1926 with the publication of the "挿込型接続器標準仕様書" (lit., "Standard for a Insertable Connector"), which became JIS C 8303. At the time, Japan was barely an industrial nation, and generally relied on imported power outlets. The leading designs up for consideration were thus the German Schuko and the American "2-prong" connectors.

In the standards, the authors actually explained their rationale for choosing the latter:

独逸の標準仕様書を参酌し、他方に於いては、現今我国にもっとも多く輸入せらる米国製品と共通性を失わざるを念とし

In considering the German standard, on the other hand, we wished not to lose compatibility with the many American products that has been imported into this country.

The United States was Japan's largest trade partner, and a technological leader; it stands to reason that American electrical appliances held a higher install base in Japan. Understandably, therefore, the Japanese standards committee settled on adopting the American plug officially, and the rest, as they say, is history.

Note: The Japanese power grid seems not to have been specifically designed to match the American one; however, like the power outlets, early electricity generation in Japan relied on imported equipment. Eastern Japan imported from the German AEG while Western Japan purchased from the American General Electric. This produced a permanent division that lasts to this day, with electricity delivered to half of Japan at 50 Hz (the same as in Europe) and to the other half at 60 Hz (same as the nominal North American frequency). This is no longer a major concern as modern devices are designed with enough tolerance to deal with either, though.

@LangLangC While not an official "standard", the two-pronged plug was by 1926 the standard US plug. Note that I am not saying the Japanese adopted an US standard, but rather that they, like NEMA in the 1940s, defined their national standard based on the prevailing US usage. Which is why the updated modern descendants of both standards are highly physically compatible.
– Semaphore♦Aug 14 '19 at 16:22

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Is Japan wide enough to have an East and West?
– RonJohnAug 14 '19 at 16:34

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@RonJohn It's more NE/SW - the dividing line cuts mostly north/south across the narrow width of the country just west of Tokyo.
– J...Aug 14 '19 at 20:48

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@gerrit: Today the two networks are connected by several pairs of back-to-back HVDC converters. It's not more of a problem than connecting two neighboring grids of the same nominal frequency that are not synchronized with each other (a situation which exists, for example, at a synchronization boundary down the middle of the continental US).
– hmakholm left over MonicaAug 15 '19 at 1:23

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@RonJohn It makes perfect sense to refer to the east and west parts of a city, which will be much narrower than Japan. And, specifically, Japan consists of a series of largely mountainous islands and most of the population lives around the coast, so the "sides" are important. Also note that Japan doesn't run north-south: the easternmost point is about a thousand miles east of the westernmost point.
– David RicherbyAug 15 '19 at 15:38

This answers primarily the first version of the question that is still evidenced in the misconception of the title, and thus the rest of this answer is still relevant to read in conjunction with semaphore's answer, which would otherwise re-inforce these misconceptions:

Q Why does Japan use the same type of AC power outlet as the US?

This is actually not the case.

Japan has not the same socket/plug system and therefore you will not find an argument for why they supposedly "adopted the US system." And neither for 'post WWII'. They only introduced a quite similar design after Edison screw plugs went out of popularity during the time-span before the war.

The US and Japan use different plugs – physically and electrically – and different standards to operate them. These differences may look small enough to group them into the same category of otherwise around the world much more different systems. But the fact that one could also stick a US blade into a Japanese socket doesn't make it a "same plug".

The Japanese plugs only look similar, but aren't fully compatible either. And it is all just historical contingency. Every country realised that the first de facto industry products that might have become any kind of 'standards' were horribly inefficient and unsafe. So they developed their own. And it took quite a long time to come to any kind of standard within any of most countries as the electrified.

Therefore the question starts from a false premise, and speculates on that: that there is deeper meaning in perceived identity when there is just similarity in pure chance developments and superficial morphollgical but untechnical categorisation…

This is clearly evidenced by early equipment not working or getting damaged if using the supposedly identical plug in the wrong region. The question would have made 'more sense' if it asked why these different plugs a grouped into the same category 'TypeA' when previously quite a lot of equipment would not work or get damaged trying to do so.

The reason why the world is now stuck with no less than 15 different styles of plugs and wall outlets, is because many countries preferred to develop a plug of their own, instead of adopting the US standard. In one sense those countries were actually right, though: the wobbly American plugs and their uninsulated prongs are almost prehistoric in terms of design and they are notoriously unsafe.

For decades, the International Electrotechnical Commission (IEC) tried to develop a universal domestic power plug, but time and again political and economic issues threw a spanner in the works. In 1986, the IEC finally presented the universal standard plug (type N) to the world, but unfortunately the initial enthusiasm had dampened. It wasn’t until 2007 that Brazil became the first country in the world to adopt type N as its standard wall outlet and plug top. The establishment of type N as the sole standard was motivated by the urge to sort out the motley collection of plugs in use throughout the country.

Many Latin-American, African and Asian countries are still in the same situation that Brazil used to be in. Standardizing on type N (or another safe and preferably widely used earthed plug system such as F, G or I, for that matter) is of course self-evident. But some countries just never learn. In 2006, Thailand deemed it necessary to develop a whole new power outlet system of its own, albeit compatible with type C, which is currently gradually being phased in. The mind boggles!

Even though the US and Japan both use plugs and outlets of type A and B they actually are not entirely the same. One of the two pins on US plugs is wider whereas in Japan both pins are the same width. This means that Japanese plugs can be used in the US, but US plugs often don't fit in Japanese outlets.

Because it is just a silly outcrop of stubborn insistence on perceived path dependencies:

Ok, maybe not every country, but with at least 12 different sockets in widespread use it sure as hell feels like it to anyone who's ever traveled. So why in the world, literally, are there so many? Funny story!

The more you look at the writhing orgy of plugs in the world, the sillier it seems.

I'd hesitate to refer to power sockets as a part of a country's culture, because they're plugs—they don't really mean anything. But in the sense that they're probably not going to change until they're forcefully replaced with something wildly new, it's kind of what they are

The Japanese plug and socket appear physically identical to NEMA 1-15. The Japanese system incorporates stricter dimensional requirements for the plug housing, different marking requirements, and mandatory testing and approval by METI or JIS.[49]

Older Japanese sockets and multi-plug adaptors are unpolarized – the slots in the sockets are the same size – and will accept only unpolarized plugs. Japanese plugs generally fit into most North American sockets without modification, but polarized North American plugs may require adaptors or replacement non-polarized plugs to connect to older Japanese sockets. In Japan the voltage is 100 V, and the frequency is either 50 Hz (East Japan) or 60 Hz (West Japan) depending on whether the customer is located on the Osaka or Tokyo grid.[50][51] Therefore, some North American devices which can be physically plugged into Japanese sockets may not function properly.

It is just the very basic form that allows for a superficial similarity and half-way easy compatibility, often with adapters or 'electrical logic' required

The superficial similarity is the "Type A" form. That does not mean they are identical, just similar.

Generally speaking, most hair dryers, curling irons, hair straighteners / flat irons and other bathroom appliances from the US and Canada will operate on 100 volt Japanese electricity, albeit less effectively (for example, the heating element won't get as hot) but it'll work, and you won't start a fire or anything. You'll need an EA6 adapter to allow the appliance to plug into the Japanese Type A wall outlet.

Other conventional appliances which are not multi-voltage compatible would require a Japan/US "booster" transformer (to change the voltage from 100 volts at the outlet to 120 volts at the plug) with a wattage capacity that is sufficient to support the wattage requirement of the appliance.

Japan is the only country on the planet where the electrical systems supply 100 volts of electricity, as opposed to 120 volts in North America and 230 volts in most of the rest of the world. Most North American appliances have a voltage tolerance of roughly +/-10%, though some equipment is more sensitive, and may have a tolerance more along the lines of 5% or even 3%. In any case, 100 volts is slightly outside the range of tolerance of most appliances. How big a deal is it? Well, it depends on the appliance, and how it's being used.

Make no mistake, even a slight undervoltage condition is not at all ideal, and not good for the health of an appliance. But undervoltage is generally considered less serious than overvoltage. You're not going to "fry" your appliance, and it's not going to catch fire or anything. In fact, it'll probably work, though less-effectively, and electronics may "act weird." And over time, undervoltage absolutely will damage and eventually destroy an appliance. That's why US/Japan "booster" transformers exist. Manufacturers wouldn't produce them if there wasn't a need for them.

If you're bringing North American appliances to Japan for an extended stay, or a permanent move, you'll want to use a booster transformer to change the voltage from 100 volts at the outlet to 120 volts at the appliance's plug, the voltage the appliance was built for. And if the appliance is particularly sensitive, or if the appliance "acting weird" is unacceptable (think medical equipment) then you need to make sure it's getting the voltage it requires. On the other hand, if you're just going for let's say a week, and you don't mind if your hair dryer doesn't get quite as hot while you're there, then you probably don't need to worry about it too much.

The electric frequency is different on either side of the Fujigawa River in Shizuoka Prefecture and Itoigawa City in Niigata Prefecture, with 50Hz in the east (Tokyo, Yokohama, Tohoku, Hokkaido) and 60Hz in the west (Nagoya, Osaka, Kyoto, Hiroshima, Shikoku, Kyushu.) Does it matter? Again, it depends on the appliance. Some appliances can handle either frequency, and for others it's not really a significant factor. Motors built for 60Hz will rotate more slowly on 50Hz, and will heat up more quickly (and over long periods of time will eventually burn out) and digital clocks and timers won't keep accurate time, they'll "run slow." Don't bother bringing an alarm clock that's not compatible with 50Hz, overnight it'll "lose" more than a full hour! But it's not going to affect your phone, tablet or laptop. And it's probably not going to be a big deal for a hair dryer for 10 minutes or so at a time.

But, again, anything that's going over there for an extended period of time, or permanently, really needs to have the voltage and frequency it was built for. And if it's a particularly sensitive piece of equipment, or something incredibly important like a piece of medical equipment that absolutely must work precisely as it's intended to, it needs to have the correct voltage. Don't chance it, give it 120 volts by way of a US/Japan booster transformer.

Japanese Wall Outlet Variance
NOW, ABOUT THOSE WALL OUTLETS

Image 1 (top-left) is a typical US / Canadian wall outlet just like the ones in your home. It's both grounded and polarized.

Grounded refers to the presence of the third, round pin in addition to the two vertical blades. Polarization refers to the left vertical blade being taller than the one on the right.

All US and Canadian wall outlets are required to be both grounded and polarized, but plugs aren't. Some plugs have the round pin while others don't, and some plugs' vertical blades are the same height while others aren't. You're probably used to that. But since the outlets are all grounded and polarized, it doesn't matter what the plug is like. They'll be able to plug into the outlet regardless.

But in Japan, virtually all outlets are non-grounded. Image 2 shows a Japanese outlet which is polarized (the left blade is taller than the right) but not grounded (there's no receptacle for the round grounding pin.) So then, what if your plug has that third, round pin? You can't plug into the non-grounded outlet. Image 3 shows a Japanese outlet that's neither grounded nor polarized. In that case, even if your plug doesn't have the third, round pin, it still can't plug in if the plug's blades aren't the same height. Most Japanese outlets are like image 2, non-grounded but polarized. But non-polarized outlets like image 3 are pretty common, especially in older buildings.

Your phone charger's plug almost certainly has just the two vertical blades, and those two blades are probably exactly the same. In that case, you're good to go, no worries. But your laptop's plug is probably polarized, with the left blade being taller. In that case, you're good to go if the outlet is like image 2, but out of luck if the outlet is like image 3. And your hair dryer's plug probably has the third, round grounding pin. In which case, you're out of luck entirely.

Fortunately, we have plug adapters for that. Our item #EA6 is neither grounded nor polarized, therefore allowing you to plug into any kind of wall outlet you might encounter in any given building in Japan, regardless of what your plug is like.

If words are too complicated to understand, perhaps it gets easier to see the difference if using pictures?

Japanese plug system from Wikipedia Japanese:

US plug system from Wikipedia:

If appliances get designed intelligently to accept and work with 100–240V and 50–60Hz etc, and thus work, with adapters, that does not mean the plug system is the same! It means nothing more but manufacturers accounting for the inherent differences to expect on a an interconnected global market.

When the different plugs within the 'same' type category were introduced you could try to plug those things from the US in Japan: that would perhaps not work at all, work badly for you, your appliance or overload the Japanese wires.

Why is the Switzerland socket different from all surrounding countries in Europe but has the exact same type as Rwanda? These actually are the very same standard! One might interpret quite a few things into that. But that probably doesn't mean one gets especially useful information out of it? If systems aren't technically the same but just look similar any connection and suspected deeper meaning is ever more absent.

For how the plugs evolved:

Edison’s plants provided light to Wall Street, New York City, and London. Edison had thought of a number of applications for this power, including a system for household wiring that used the existing piping of the house as its framework. However, there was no convenient way of tapping into this power for anything other than lighting. Amateur inventors came up with a number of appliances that had to be directly wired to the household system, including Harvey Hubbell, who designed a “Separable Attachment Plug” that connected directly into a light socket. This plug had to be wired to the appliance, but the user would not have to deal with live wires connected to the house. He then improved upon his own design by making the plug itself able to separate: one portion of the plug could be left in the socket, while the other was a two-prong plug that could be separated from the socket plug.

After Hubbell’s innovative plug, the next advancement came from a man named Philip F. Labre in 1928. In order to reduce electrical shock as a result of a short circuit (an undesirable connection), he added a third ground prong to the plug to be inserted into a third hole in the socket. When a person unknowingly creates a short in a circuit with an electrical plug, his body becomes the only path from the live wire of the plug to the ground. Labre added the third prong as an alternative “path of least resistance” to bypass the person. This means that instead of flowing through you, the electrons flow through the ground prong to the earth. 4.
Labre’s outlet is the model for the modern outlet. Aside from the ground prong, there are two prongs that connect to the live and neutral wires. The live wire carries current into the appliance, while the neutral wire carries current back to the electric panel, completing the circuit [4:3]. Without the grounding, Labre’s outlet would be called “unpolarized” because the two identical prongs of the plug may be inserted into the outlet in one of two ways. A “polarized” outlet allows the plug to be oriented in only one way (“Household Wiring”). Worldwide, some plugs have different-sized prongs (like in the US), some are set at different angles (like in some European countries), and some can have completely different shapes (like in Japan).
–– Robin Hartman: "A Powerful History: The Modern Electrical Outlet", Illumin Electrical Engineering, Issue II, Lifestyle, Volume X, June 19th, 2008.

Japan uses flat blade plugs and sockets according to Japanese JIS 8303 standard. The 15 and 20 Amp types are similar to U.S. NEMA configurations. …

Japan follows the North American pattern flat blade configurations for 15A-125V (left), 20A-125V (middle) plugs. The Japanese 20A-125V without earth connection (right) has an unique shape for the neutral (W) pin. …

Socket (13) and plug (14) rated at 15A - 125V with a U.S.-style twist lock mechanism. However these Japanese models are incompatible to NEMA L5-15 plugs and sockets. Japanese plugs have a slightly different pin position and the earth pin does not have a hook.

A Japanese site explains it thusly:

Different countries have different standard voltages and electrical sockets. Some of the gadgets and devices we have may not work in some countries if the voltage, plugs, and sockets are not compatible.

The U.S. developed the power delivery system along with the modern electric plug. Other countries did not find the U.S. standards (60 Hz, 110 V and their plug system) as efficient for the power supply being utilized.

Each country began improving on what they thought would be the best way to deliver electricity that is safe and customized to their standards. This resulted in different countries having different plugs and sockets.

Since these original designs and their variations or improvements around the world date to the 1920s and 1930s, with further developments on that front hampered by another front – it follows that at the time of adoption Japanese did not receive any post-war aid but decided that absent a whole domestic 'invented here' solution some parts of the US design that became standard in the US after the Edison screw finally disappeared for appliances other than light in 1931 was 'good enough', other aspects not, and thus these were changed.

In case of Japan again: the literally first search engine hit for pre war Japanese radio,

In the Meiji period, 50Hz (Hertz) generators were imported from Germany in the Kanto region, and 60Hz (Hertz) generators were imported from the United States to the Kansai region. And there are two frequencies.

Some household electrical appliances that use motors, transformers, and electronic timers with built-in equipment are based on this frequency, so moving parts between the 50Hz and 60Hz areas requires replacement of parts and adjustment of equipment. There are cases.

Precautions when using electrical products overseas
Some things can't be used overseas
Japan's voltage is 100V and 200V, and overseas is 110V, 120V or 220, 240V. If the voltage is different, the electrical product may break.

When used in a different location, the electricity is flowing to properly check or the electrical outlet display confirmation trying.

There are various outlets.
If the frequency varies from country to country, the voltage and the shape of the outlet will also vary. Unless it is an electrical product that says it can be used anywhere overseas, there is not much you can use Japanese products as they are.

When you go abroad, there are small capacity transformers that adjust the voltage and plug adapters that match the shape of the outlet at travel goods dealers and rental stores.

If you don't know why Japan chose its current standards, you should not be answering this question.
– axsvl77Aug 14 '19 at 7:56

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The point is that it's not a coincidence that I can plug something I bought in Japan in a US socket and vice versa.
– spacetyperAug 14 '19 at 8:48

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Yes, true. Of course. AC/DC converters are pretty tolerant. Nevertheless, Japan for some reason adopted this standard, This answer does not address the reason. I'm not sure why I'm so outraged by this answer honestly. Might be because it is making very semantic argument against a decent question by a new user.
– axsvl77Aug 14 '19 at 9:57

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A big part of the reason that you can plug in electronics from the US and it "just works" is because most modern electronics comes with power bricks that accept voltage ranges from 100-240 volts. The reason for that is that it is cheaper to have one power brick for the world and only vary the adapters. Anything that doesn't use a transformer won't transfer so well. It's just that such things are rarer and rarer in the modern world
– Gort the RobotAug 14 '19 at 13:31

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Your "Japanese plug system" image looks to me like a polarized NEMA 1-15, a NEMA 5-15, a NEMA 5-20, and something that can accept a NEMA 6-15 plug or plug into a NEMA 6-20 socket. Your "pre war Japanese radio" looks like it's got a Hubbell plug (equivalent to unpolarized NEMA 1-15). If you want to make the case that Japan doesn't use a US-compatible plug system, you'll need better evidence.
– MarkAug 14 '19 at 21:18